System for separating magnetic susceptible particles



June 27, 1961 P. E. CAVANAGH EI'AL 2,990,124

SYSTEM FOR SEPARATING MAGNETIC SUSCEPTIBLE PARTICLES Filed Aug. 16, 1957 INVENTORS CARL W. HEDBERG PATRICK E. CAVANAGH BYHMKAW AMA f TORNEYS United States Patent 2,990,124 SYSTEM FOR SEPARATING MAGNETIC SUSCEPTIBLE PARTICLES Patrick E. Cavanagh, Oalrville, Ontario, Canada, and Carl W. Hedberg, Bound Brook, N.J., assignors to Research-Cottrell, Inc., Bridgewater, N.J., a corporation of New Jersey Filed Aug. 16, 1957, Ser. No. 678,468 8 Claims. (Cl. 241-24) This invention relates to a new and improved system for separating and concentrating magnetic susceptible materials.

It is a particular object of the present invention to provide a system for concentrating magnetite bearing low grade ores to provide a premium ore containing 60% or better iron content.

A process which is economically suited to the beneficiation of low grade iron ore must treat large tonnages of ore at low overall costs in equipment which is relatively inexpensive to manufacture, rugged and dependable in operation, substantially automatic in operation and which substantially eliminates any danger to the health of the operators.

It is, therefore, a particular object of the present invention to provide such a system principally adapted for concentrating low grade iron-containing minerals having relatively high magnetic attractabilities such as magnetite (Fe O Franklinite ((FeMnZn) O. (FeMn O ilmenite (FeT O and the like. These minerals have appreciably higher magnetic attractabilities than, for example, the feebler magnetic susceptible iron-containing minerals such as hematite (Fe O It is a further object of the present invention to provide a system whereby ores containing, for example, about 25% iron by weight in the form of acid soluble iron may be economically concentrated to contain upwardly of 60% total acid soluble iron.

These and other objects and advantages of the present invention are provided by a system which generally comprises grinding the iron-containing mineral in a suitable dry type mill, passing a stream of air through said mill to continuously remove from the mill material ground therein suspended in said air stream, passing the ground material to a classifier, separating the ground material in the classifier into fine and coarse fractions, returning said coarse fraction to the mill, subjecting said stream of air containing the fine fraction to a magnetic field to separate said air suspended particles into a fraction containing substantial quantities of magnetic susceptible material and into a fraction substantially depleted of magnetic susceptible material and thereafter removing said suspending air from said fractions and by the system which comprises grinding the iron-containing minerals in a suitable dry-type mill, passing a stream of air through the mill to continuously remove from the mill material ground therein, passing the suspension of ground material to a classifier, separating the ground material in the classifier into fine and coarse fractions, passing the coarse fraction through a magnetic separator to separate said coarse fraction into a magnetic susceptible portion and a -non-magnetic portion, returning the magnetic susceptible portion to the mill, removing the non-magnetic portion from the system, subjecting the stream of air containing the fine fraction to a magnetic field to separate said air suspended particles into a fraction containing substantial quantities of magnetic susceptible material and into a fraction substantially depleted of magnetic susceptible 2,990,124 Patented June 27, 1 961 "ice material and thereafter removing such suspending air from said fractions.

These and other objects and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description of the system with reference to the drawing showing a flow sheet of a preferred embodiment of the process of the present invention.

The drawing shows a flow sheet of a preferred system embodying the present invention wherein 10 is a grinding device which may be of the ball, rod, tube or autogenous type. Very satisfactory grinding of the raw material fed into the mill as indicated by arrow 12 has been accomplished through the use of conventional air sweep mills such as those shown in the following patents: Weston, 2,555,171; Weston, 2,566,103; Hardinge, 1,872,036.

As to be more fully described hereinafter a portion of the air stream adapted to suspend and carry ground material from the mill is supplied from the ambient atmosphere and a portion is recycled from later stages of the system of the present invention through conduit 14. The air entering the inlet 16 of the mill picks up the material reduced in size within the mill 1t) and passes therefrom through conduit 18 carrying said material in suspension. As will become apparent to those skilled in the art, from the further description of the present invention, the size and concentration of the particles carried in the air stream from the mill 10 through conduit 18 will depend to a large extent on the flow rate of air through the mill.

The air stream carrying the suspended particles is then directed to a conventional classifier designated 20 wherein the particles suspended in the carrier air stream are separated into at least two fractions. One of the fractions comprises a substantial portion of fine material and the other comprising primarily the coarse material. The classifier 20, like the grinding mill 10, does not comprise a specific structural element embodying applicants invention and details of the classifier will not be further described herein. However, classifiers such as shown in patents: Case, 601,728-Schutz, 1,305,413--Hardinge, 2,381,954have proved to be highly satisfactory. It is pointed out that while the classifiers shown and described in these patents give very satisfactory results, it will be apparent to those skilled in the art that other forms of mechanical classifiers may be readily employed in the system.

From classifier 26 several courses may be followed within the scope of the present invention for further treatment of the coarse and line fractions of ground material. As shown in the flow sheet the coarse fraction is directed from classifier 20 to a screen-type classifier 22 from which large particles, for example, +20 mesh, are removed through conduit 24 to the inlet 26 of an endless belt-type elevator 28. The elevator 28 conveys the large particles from the screening device 22 to a conduit 30 which discharges the material into the inlet 16 of the grinding device 10. The portion of the coarse fraction of ground material passing through the screen of screening device 22 is conveyed by conduit 32 to one or more magnetic separators 34. A further description of the separators 34 will be found in a United States application filed on even date herewith in the name of P. E. Cavanagh and entitled Magnetic Separator.

The coarse fraction of material, after passing through the screening device 22, is subjected to a relatively strong magnetic field in passing through one or more separators 34. The separators 34 eject the coarse, magnetic susceptible material therefrom through conduits 36 and 36 to conduit 38 thence to an endless belt conveyor 40- which conveys the coarse, magnetic susceptible material into hopper 26 of elevator 28 where the particles, may be mixed with the non-concentrated coarse particles from conduit 24 of screening device 22 and both the magnetic susceptible and non-magnetic particles, may then be conveyed to the inlet of the mill for regrinding or where the magnetic susceptible material removed by separators. 34 is very high in iron content, such material may beremoved from the system at this point and stored as concentrated magnetic iron ore.

The non-magnetic material and those having very weak magnetic susceptibilities such as hematite are discharged from the magnetic separators 34 through conduits 42, which as shown in the flow diagram may be serially arranged, thence to a further conveyor 44. The material deposited on the conveyor 44 from conduits 42 will be considered in the present description as tails and are removed by the conveyor 44 from further treatment in the prment system.

The removal of coarse non-magnetic materials by magnetic separators 34 substantially reduces the power requirements of the system as it eliminates the need for grinding to a fine mesh large particles of material which contain no or very little magnetic iron ore.

The fine fraction of ground material from mechanical classifier 20 passes through conduits 46 to one or more novel magnetic separators designated 48. Within the magnetic separators 48 the fine fraction is segregated into a concentrated magnetic susceptible portion which is discharged through conduits 50 to conduits 52 thence to further conveyor means 54 where the material is stored or subjected to further treatment as the case may be. The non-magnetic susceptible portion of the fine fraction passing from the magnetic separators 48 through conduits 56 will be designated tails or middlings and passed to further serially arranged magnetic separators 34' substantially identical in form to the magnetic separators 34 which act on the coarse fraction of the ground material from screening device 24 hereinbefore described. Where further treatment of the tails from the magnetic separators 48 is found to be desirable the plural magnetic separators 34' remove further amounts of magnetic susceptible material which passes through conduits 58 to primary conduit 60 thence to the conveyor 40 where the particles are mixed with the magnetic susceptible material passing from conduit 38 of magnetic separators 34 and then into the hopper 26 for transportation to the inlet 16 of the mill as hereinbefore described. In general it has been found that removal of 90% of the suspending air permitting only 10% of the air to pass with the fine fraction to the separators 34' has been found to give very satisfactory results. For example, where 30 cubic feet of air is required to suspend each pound of fine fraction of ground material, the volume of air is decreased to three cubic feet per pound of dust prior to the passage of the fine fraction through the helical magnetic separators, and satisfactory results are obtainable when from about 80 to about 95% of the suspending air stream is removed at this point. The non-magnetic particles from the plural separators 34 are discharged through conduits 62 and 64 to conveyor 44 for removal from the system of the present invention.

A portion of the air stream containing particularly fine suspended material is removed by the feeding devices for the separator units 48 and passes through ducts 66 to a blower 70 so that the fine fraction of material passing to the separator units 48 does not contain more than 60% 325 mesh material. Downstream of the blower 70 the air stream from the particle distributing devices of separators 48 is divided by valve means 72 into either conduit 14 or conduit 74. Conduit 14 provides a portion of the feed air for the air swept mill 10 while conduit 74 directs the gas stream containing very fine dust to a conventional cyclone type mechanical separator generally designated 76. 7

The mechanical separator 76 may be of the type shown in United States patent application Serial No. 633,359 filed on January 9, 1957. While mechanical separators disclosed and claimed in said application provide very satisfactory dust removal, it will be apparent that other forms of mechanical or electrostatic or mechanical and electrostatic dust collectors may be employed to remove the remainingadust in theair stream flowing through conduit 74.

Where the dust in conduit 74 is very fine the discharged air'from the mechanical collector 76 through conduit 78 may be directed through a further blower 80 and thence through conduit 82' to a conventional electrostatic precipitator 84 such as the type shown in United States Patents 2,631,686 and 2,717,053 or other conventional gas cleaning or scrubbing apparatus. The clean gas stream from the electrostatic precipitator 84 exhausts to atmosphere through stack 86 while the agglomerated and collected dust particles are removed from the system through outlet 88.

From the foregoing description it will be seen that a complete system, for concentrating magnetic susceptible material, such, as low-grade magnetite ores, from mine size minerals entering-the system at 12, to the final cleaning of the suspending air stream and the removal of tails and concentrated magnetite is provided by the present invention.

Operation of the new and improved system of the present invention will be more clearly apparent from the following detailed description of an example of operation thereof.

Example About 7000 pounds of Oceanic iron ore was fed toa mill having a diameter of 5% feet and rotated at 29 rpm. A draft of2 inches of water through the drum carried ground particles having; the following screen A sample of this ground material analyzed 29% acid soluble iron. The ground material was then passed through a mechanical classifier (20) wherein it was separated into fractions above and below mesh in particle size. The coarse fraction of the material was passed over a screening device (22) and 73% of the coarse fraction was directed to magnetic separator 34. The tails from the magnetic separator 34 were then passed through a further magnetic separator. The total tails comprised about 20% of the total weight of the feed. The combined middlings and concentrates from the two magnetic separators 34, comprising about 25% of the total weight of the coarse fraction passed to the magnetic separators, were recycled to the grinding mill.

The fine fraction of material was then passed in a stream of air having about 30 cubic feet of air per pound of fines to separators 48. About 93% of the total fines was collected in a plenum chamber (20) below cyclone separators (182) and directed to the first stage of the helical concentrators. The concentrates removed in the first stage of the helical concentrators comprised about 28% of the feed which contained about 68% total iron. The tails. from the first concentrator stage were then passed through the succeeding stages and the concentrate withdrawn from the successive stages comprised about 7 of the'total weight of the feed and had an iron contentof 49%. The combined tails from the separators (48) were then passed. through further magnetic concentrators (34'). and the; middlings therefrom were recycled to the mill. The tails from the magnetic conc'entrators (34') comprising 31% of thetotal fe'ed' were discharged to waste. The concentrates finally recovered comprised about 36% of the total feed weight and con-' tained 63% acid soluble iron as compared to the original content of 29% in the feed to the grindingmill.

From the foregoing description, it will be seen that the present invention provides a new and useful dry system for concentrating low grade magnetic susceptible iron ores and apparatus for carrying said process into operation.

While the invention has been described with reference to a particular flow pattern for materials, it will be appreciated that various changes may be made therein and the number of separating stages in each of the separating units may be variously modified depending on the particular type of ore being concentrated.

We claim:

1. A process for separating magnetic susceptible particles from bulk material comprising dry grinding magnetic-containing mineral, passing a stream of air through the grinding means to continuously remove therefrom material ground therein suspended in said air stream, passing the air stream and suspended ground material to a classifier, separating the air suspended ground material into fine and coarse fractions, returning said coarse fraction to the grinding means, subjecting said stream of air containing the fine fraction to a magnetic field to separate said air suspended particles into a fraction containing substantial quantities of magnetic susceptible material and into a fraction substantially depleted of magnetic susceptible material and thereafter removing said suspending air from said fractions.

2. A system for concentrating magnetic iron-containing minerals which comprises dry grinding the iron-containing minerals, passing a stream of air through the dry grinding means to continuously remove therefrom material ground therein, passing the air suspension of ground material to a classifier, separating the air suspended ground material in the classifier into fine and coarse fractions, subjecting the coarse fraction to a magnetic field to separate said coarse fraction into a magnetic susceptible portion and a non-magnetic portion, returning the magnetic susceptible portion to the grinding means, removing the non-magnetic portion from the system, subjecting the stream of air containing the fine fraction to a magnetic field to separate said air suspended particles into a fraction containing substantial quantities of magnetic susceptible material and into a fraction substantially depleted of magnetic susceptible material and thereafter removing such suspending air from said fractions.

3. A system for concentrating magnetic iron-containing minerals which comprises dry grinding the iron-containing minerals, passing a stream of air through the dry grinding means to continuously remove therefrom material ground therein, passing the suspension of ground material to a classifier, separating the air suspended ground material in the classifier into fine and coarse fractions, subjecting the coarse fraction to a magnetic field to separate said coarse fraction into a magnetic susceptible portion and a non-magnetic portion, returning the magnetic susceptible portion to the grinding means, removing the non-magnetic portion from the system, removing about 90% of the suspending air from the fine fraction, subjecting the remaining stream of air containing the fine fraction to a magnetic field to separate said air suspended particles into a fraction containing substantial quantities of magnetic susceptible material and into a fraction substantially depleted of magnetic susceptible material and thereafter removing such suspending air from said fractions.

4. A system for concentrating magnetic iron-containing minerals which comprises dry grinding the ironcontaining minerals to particle sizes in the range of from about 20 mesh to about +325 mesh, passing a steam of air through the dry grinding means to continuously remove therefrom ground material therein,'-passing the suspension of ground material to a classifier, separating the air suspended ground material therein into a fine fraction above 150 mesh and a coarse fraction below 150 mesh, subjecting the coarse fraction to a magnetic field to separate said coarse fraction into a magnetic susceptible portion and a non-magnetic portion, returning the. magnetic susceptible portion to the mill, removing about of the suspending air from the fine, fraction, subjecting the remaining stream of air containing the fine fraction to a magnetic field to separate said air suspended particles into a fraction containing substantial quantities of magnetic susceptible material and into a fraction substantially depleted of magnetic susceptible material and thereafter removing said suspending air from said fractions.

5. A system for concentrating magnetic iron-containing minerals which comprises dry grinding the iron-containing minerals into particle sizes of the following order:

Percent passing a stream of air through the grinding means to continuously remove therefrom material ground therein, passing the suspension of air suspended ground material to a classifier, separating the ground material therein into fine and coarse fractions, subjecting the coarse fraction to a magnetic field to separate said coarse fraction into a magnetic susceptible portion and a non-magnetic portion, returning the magnetic susceptible portion to the grinding means, removing the non-magnetic portion from the system, subjecting the stream of air containing the fine fraction to a magnetic field to separate said air suspended particles into a fraction containing substantial quantities of magnetic susceptible material and into a fraction substantially depleted of magnetic susceptible material and thereafter removing such suspending air from said fractions.

6. A system for concentrating magnetic iron-containing minerals which comprises dry grinding the iron-containing minerals into particle sizes of the following order:

passing a stream of air through the grinding means to continuously remove therefrom material ground therein, passing the suspension of ground material to a classifier, separating the air suspended ground material therein into fine and coarse fractions above and below 150 mesh in particle size, subjecting the coarse fraction to a magnetic field to separate said coarse fraction into a magnetic susceptible portion and a non-magnetic portion, returning the magnetic susceptible portion to the grinding means, removing the non-magnetic portion from the system, removing about 90% 0f the suspending air from the fine fraction, subjecting the remaining stream of air containing the fine fraction to a magnetic field to separate said air suspended particles into a fraction containing substantial quantities of magnetic susceptible material and into a fraction substantially depleted of magnetic susceptible material and thereafter removing such suspending air from said fractions.

7. The invention defined in claim 6 wherein said fine 7 fraction; contains: fromtahout; 1010: about; 40 of 325 mesh:- particles;

8; A system;forconcentratingrmagnetic iron-contain. ing; minerals comprising, dry grinding; the? iron-containing minerals, passing a stream of air through the grinding means to-continuously remove therefrom, material ground therein, passing. the suspension of ground material to a classifier,1separating; theair suspended. ground; material therein. into. line and coarse fractions, subjecting the coarse fraction to amagnetic field of at least about 70.0 gauss to separate: said coarse'fraction: into a magnetic susceptible portion and a non-magnetic portion, return-t ing, the magnetic susceptible portion. to the grinding means,,removing.the non-magnetic portion fromithe sys tem, subjecting-the stream of'air containing the fine fracs tion, to a magnetic field of'from about 700- to about 1000 gauss'to separate said air'suspended particles into a frac'- tion containing substantial quantifies of'magnetic suscep- 8 tible: material. and: into a fraction substantially depleted of magnetic:susceptiblematerial; and thereafter: removing said suspending, air fromisaidt fractions.

References Cited in' the-file of thispatent UNITED STATES" PATENTS 704,010 Edison July 8, 1902 1,905,780 Ahlmann Apr. 25, 1933 2,045,098 Payne Iune23, 1936 2,078,513 Stea-rns Apr. 27, 1937 2,175,484 Reesretal Oct, 10, 1939 2,269,912 Ladoo et a1. Ian. 13, 1942 2,307,064 Patterson Jan. 5, 1943 2,674,413 Weston Apr. 6, 1954 2,692,677 Bosqui Oct. 26, 1954 2,699,871 Stem Jan. 18, 1955 

